Sector scanning servo-motor controlled apparatus

ABSTRACT

A beacon sector scanning apparatus includes a light source mounted to a fixed base and projecting a beam of light upwardly to a head assembly comprising a rotating head with a pentaprism for projecting the light beam outwardly from the head. The head assembly further includes a fixed support removably attached to the base and to which the head is rotatably mounted. A reversible variable speed motor and means coupling the motor to the head are positioned on the support. An electrical circuit provides a drive signal for the motor and responds to a control signal from detection means positioned on the support adjacent the head which is actuated by actuation means positioned on the rotating head to periodically reverse the motor direction. The magnitude of current supplying the drive motor is adjustable to vary the arc of oscillation of the head over the selected predetermined time period to provide a sector scan with an adjustable arc.

United States Patent [191 Dufendach Dec. 3, 1974 SECTOR SCANNING SERVO-MOTOR Primary Examiner-T. E. Lynch CONTROLLED APPARATUS Attorney, Agent, or Firm -Price, Heneveld, l-luizeng 75 Inventor: William R. Dufendach, Grand Cooper Rapids, Mich.

[73] Assignee: Vari-Tech Company, Grand Rapids, ABSTRACT Mlch' A beacon sector scanning apparatus includes a light [22] Filed: Nov. 9, 1972 source mounted to a fixed base and projecting a beam of light upwardly to a head assembly comprising a ro- [21] Appl' 305105 tating head with a pentaprism for projecting the light beam outwardly from the head. The head assembly [52] US. Cl. 318/627, 318/681 further includes a fixed support removably attached to [51] Int. Cl G05g 5/00 the base and to which the head is rotatably mounted. [58] Field of Search 318/627, 626, 678, 681, A reversible variable speed motor and means coupling 318/331, 608 the motor to the head are positioned on the support. An electrical circuit provides a drive signal for the [56] References Cited motor and responds to a control signal from detection UNITED STATES PATENTS means positioned on the support adjacent the head 2 588 743 3 1952 McCallum 318/608 Y which is actuated by aftuation means positioned 2:780:763 2/1957 Henwig 318/34] X the rotating head to periodically reverse the motor di- 3,427 520 2/1969 oppendaml 318/678 X rectlon. The magnitude of current supplying the drive 3,440.505 4/1969 Nielsen I 27 motor is adjustable to vary the arc of oscillation of the 3,541,419 11/1970 Jasons 318/627 head over the selected predetermined time period to 3,665,282 5/1972 Skehan 318/627 provide a sector scan with an adjustable arc. 3,668,496 6/1972 I Markowits et al. 3l8/626 X 10 Claims, 7 Drawing Figures SHEET 1 [IF 2 PATENTEL DEC FIG 6 VARIABLE DC ONE SHOT CMONOSTABLE) MULTIVIBRATOR d FI G4A- SECTOR 7 SCANNING SERVO-MOTOR CONTROLLED APPARATUS BACKGROUND OF THE INVENTION The present invention relates to a beacon sector scanning apparatus and-particularly to improved means for oscillating a scanning beam through an adjustable arc.

The use of laser light sources has become increasingly popular for surveying and general construction work where the projected light beam forms a reference line which can easily be detected. Recently, a system has employed a generally vertically oriented laser with the beam deflected at right angles by a rotating mirror to project the beam in a plane as the mirror continuously rotates. The-reference plane so formed can be used, for example, in the construction of ceilings or other horizontal planar structures.

It is desirable in some applications to project the beam of the latter type of laser alignment devices over a preselected arc instead of a full 360'rotation. To accomplish such a projection, a sector scanning control must be provided to oscillate the mirror head over a desired are which then projects a reference plane in the form of .an arcuate segment. Sector scanning is relatively wellknown in the radar art where relatively complex servomechanism and control circuits have been employed to provide sector scanning of antennas.-

Other electromechanical means have been provided to automatically'reverse the direction of scan of radar antennas after a predetermined arc has been swept. U.S. Pat. No. 2,789,261 issued on Apr. 16, 1957, to F. J. Hoffman, is representative of such an electromechanical system for use in radar installations and which employs adjustable time delay circuits to achieve sector scanning for a desired arcuate segment. Although these and other electromechanical systems have been useful in radar installations, their application to relatively small, lightweight laser alignment devices is not feasible due to the required portability, size, and cost restrictions of such instruments.

There exists, therefore, a need for improved sector scanning control means which employ miniaturized components which are relatively inexpensive and can be adapted for use with laser aligning instruments for projecting a sector scanning reference beam which can be adjusted as required by particular needs.

SUMMARY OF THE PRESENT INVENTION the rotatable head and its fixed support to reverse the direction of rotation when the head reaches a reference position.

Apparatus embodying the present invention includes a base assembly in which a light source is mounted and to which is removably mounted a head assembly including a rotatable light reflecting head and support which projects a light beam from the source outwardly in an adjustable arcuate segment. Detection and actuation' means are positioned on the head and support such 1 that as the head moves to a reference position, an electrical control signal is generated which is employed to reverse the direction of movement of the rotary head. A control circuit receives the controlsignal and applies a drive signal to a head rotating motor and actuates the motor to rotate the'head away from the reference position for a predetermined time whereupon the control circuit reverses the direction of rotation of the head until it returns to the reference position. The drive energy applied to the motor is variable to vary the speed of the motor and thereby determine the arc subtended by the oscillation scanning head over the preselected period of oscillation.

It is an object of the present invention to provide an improved beacon sector scanning apparatus.

An additional object of the present invention is to provide a beacon sector scanning apparatus having a fixed period of oscillation for variable scanning arcs.

Another object of the present invention is to provide a compact, lightweight and inexpensive sector scanning control including detecting and actuation means for periodically reversing the direction of the scanning beam.

These and other objects of the present invention will become apparent upon reading the following description thereof together with the accompanying figures in which:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of the head assembly with the protective cover removed;

FIG. 2 is a top plan view of the head assembly;

FIG. 3 is a fragmentary cross-sectional view of the head assembly and the base assembly to which it is removably mounted taken along the section plane III-Ill of FIG. 2; I

FIG-4A is a perspective view of the top of the fixed support for the rotating head;

FIG. 4B is a perspective view of the rotating head shown in an inverted position;

FIG. 5 is a diagram of the control system showing the mechanical portions in pictorial form and the electrical components partly in block and schematic form; and

FIG. 6 is a schematic circuit diagram of the control circuit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT tric intermediate mounting sleeve 56, a second annular space 58 filled with the same insulating material, and

an inner concentric sleeve 60 made of an electrically conductive material. A laser 62 or other suitable light source is fixedly mounted to the base assembly 50 in a plate 40 is secured.

(shown schematically in FIG. 3), exits the open end 64 of sleeve 60, and is projected to the head 20 through a hollowshaft 48 (FIG. 3).

The head assembly is removably mounted over the end of the upwardly projecting base assembly 50 by means of the generally hollow shaft housing 32 forming a portion of the fixed support assembly 30. The shaft housing 32 includes a circular recess 34 formed partially through the housing from the bottom and into which the end of cylindrical tube 52 fits and snugly seats against an annular shoulder 35 at the end of recess 34. The shaft housing 32 is fixedly attached to the outer cylinder 52 of the base assembly 50 by means of suitable set screws (not shown). This permits the head assembly 10 to be mounted to the base assembly at a.

cal connection between the base and the head.

' Extending through the shaft housing 32 concentric with recesses 34 and 34' is a shaft receiving aperture 37. Asuitable bushing 38 is fitted along the interior surface of aperture 37 and permits the rotation of the hollow cylindrical shaft 48 which has a flared bottom end 49. A thrust washer 39 is seated against shoulder 36 between the flared end of shaft 48 and housing 32. The top of the shaft housing 32 includes an upwardly projecting rim 31 with a shoulder 33 to which a mounting Mounting plate 40 includes a central aperture 42 therein whichpermits the plate to be fitted over the rim of the shaft housing and secured thereto by means of bolts 44 spaced around aperture 42. Mounted below and at one end of mounting plate40 is a circuit board clamped to the top end of the shaft.

On the top of housing 22 is mounted a pentaprism 24 of conventional design to reflect the light beam 65 (in the direction indicated by the arrows in FIG. 3) outwardly through the front of the pentaprism. A lens hold-down clamp 25 isemployed to hold the lens in place on the housing 22 such that the'beam receiving surface of prism 24 is in communication with the open top end of aperture 21. A wedge lens assembly 26 is positioned on the exit face of the pentaprism 24 to provide the slight amount of refraction necessary to project the beam from laser 62 from the head assembly 20 orthogonal to the axis of the input beam 65. The mirror structure,.including the wedge lens and pentaprism, is protected by a suitable cover 28 with an aperture 29 aligned with lens 26 to permit the passage of light outwardly from the head.

Drive motor 80 is coupled to the rotating head 20 by means of a drive belt 85 (FIGS. 1 through 3) which extends around the shaft -83 of the drive motor and around a vertical peripheral wall 27 at the lower end of housing 22 as seen in these figures. Belt 85 may comprise a pair of O-rings as shown or beof other suitable design. As the motor shaft 83 rotates, the head 20 and shaft 48 rotate within the shaft aperture 37 of the fixed housing 32 thereby permitting rotation of the head as sembly 20 with respect to the fixed base 50 and support assembly 30. To permit manual adjustment of the position of the head, the end of motor shaft 83 extends through an aperture in thecover 75 and has a knob 82 mounted thereon.

Referring now to FIGS. 3 and 4, the actuation means and detection means positioned on the rotating head and fixed support assembly, respectively, is shown. The

" detection means comprises a reed switch 90 which is pled to the rotating head as described below. A twopiece protective cover 75 (FIGS. 2 and 3) isfitted over the mounting board40 and extends around the sides and bottom of the head assembly to protect the assembly;

ing 22 (FIGS. 3 and 4B) having a first annular recess 15 formed from the bottom and terminating at a shoulder- 16, a second smaller diameter annular recess 17 extending from shoulder 16 and tenninating at a shoulder 18, and a shaft receiving aperture 21 extending through the housing. Housing 22 is removably coupled to the top end of shaft 48 by means of set screws (not shown) to provide rotatable mechanical coupling between the head'20 and fixed support 30. Housing 22 is slidably supportedon the rim 31 of fixed housing 32 by means of a thrust washer 23 seated against shoulder 18 of recess l7 and interposed between the housings. The flared end 49of shaft 48 prevents the head from being fitted within a recess 41'formed in the mounting plate 40 adjacent rim 31 of housing 32 as seen in FIG. 4A.- Switch is held securely in place by means of a bonding adhesive 41' such as a silicone rubber compound and illustrated schematically in the figures. Electrical leads 92(FIG. 6) couple the switch 90 to circuit board 70 and to the control circuit thereon. The actuation means comprises a permanent magnet 95 which is bonded to the shoulder 16 of annular recess 15 formed in the lens housing 22 by means of a suitable bonding agent. Its placement is shown in FIGS. 3 and 4B. The magnet is sufficiently strong and positioned on housing 22 such that as the magnet approaches the reed switch when thehead rotates, the field from the magnet actuates the switch to close its contacts before themagnet is directly over the switch..As explained below, this motor.

By employing the magnet and reed switch and positioning them in this manner, the rotatinghead is prevented from sweeping past the magnet due to its inertia. By placing the reed switch with its electrical contacts on the fixedmounting plate 40, the electrical leads therefor can easily be coupled to the control circuit on circuit board 70 The actuating means (i.e., magnet 95) requires no external connections such as slip rings between the rotating head and the support assembly. This combination provides, therefore, an inexpensive and reliable means for generating a control signal at a reference position of the head during its rotation. Since the reed switch is fast acting, it is particumentary switch closure is employed to reverse the molarly suitable for developing a reversing signal for the rotating head.

DC power for the drive motor 80 and the control circuit is coupled to the head assembly by means of the inner conducting cylinder 60 of the base assembly 50. Cylinder 60 is coupled to a 12 volt supply such as a storage battery or the like which provides portability for the instrument. The spring contact arm 46 mounted to the recess 34' above shoulder 35, as shown in FIG. 3, contacts the conducting cylinder 60 once the head assembly is positioned on the base thereby coupling the 12 volt power to contact 46 which, in turn, is coupled to the control circuit by a conductor extending through housing 32 and connected to circuit board 70. The control circuit 100 which supplies drive energy to motor 80 and is controlled in part by the actuation of reed switch 90 for operating the beacon over a selected arcuate segment, is shown in block and schematic form in FIG. 5.

Referring now to FIG. 5, the control circuit 100 comprises a monostable multivibrator 110 having an input terminal coupled to reed switch 90 and output terminals coupled to electronic switches 140 and 150 represented in the FIG. 5 circuit diagram as double-pole single-throw switches. Switches 140 and 150 couple the drive motor 80 to a variable D.C. (direct current) supply 160 and are simultaneously operated by multivibrator 110 to reverse the current flow through the motor 80.

Monostable multivibrator 110 is normally in its stable state to drive motor 80 in a first direction. It is triggered into its unstable state by the momentary actuation of reed switch 90 whenmagnet 95 on the rotating head approaches switch 90 and reverses the drive to motor 80. The relaxation period of multivibrator 110 determines the period of time the motor 80 will be reversed to rotate head 20 in an opposite second direction. Although this period can be adjusted, it is generally preset. The arc subtended by the beacon is controlled by controlling the magnitude of the voltage from supply 160 applied to motor 80. This, in turn, drives the motor at a presettable speed and, depending upon the current selected, the scanning arc for the preset relaxation time of the multivibrator l10'provides a sector scan of a de sired angle. The specific circuit for providing the monostable multivibrator 110, the electronic switches 140 and 150 and the variable D.C. supply 160 are 'shown together with the controls for the circuit in FIG.

As noted above, the electrical circuit shown in FIG. 6 can be mounted on the circuit board 70 shown in FIG. 3 and forms an integral portion of the head assembly 10. 12 volt DC. power for the circuit is supplied to the spring contact 46 as shown in FIGS. ,3 and 6. A

three-position double-pole switch 112 has two sections 112a and 112b each having rotate, off and scan positions as the switch is moved clockwise. The switch is shown in its scanning position in FIG. 6. In the continu-' The multivibrator comprises a pair of interconnected NOR gates 115 and 120. Gate 115 includes input terminals 113 and 114 and an output terminal 116. Operating power is supplied to a power supply terminal 117 through a protection diode 125 and voltage regulating means including a series dropping resistor 126 and a zener diode 127 coupled from the junction of resistor 126 and terminal 117 to a common or ground lead 130.

NOR gate 120 includes input terminals 121 and 122 and output terminal 123 and a ground return terminal 124 which is coupled to line 130. Gates and are coupled in a multivibrator configuration by means of a series RC network comprising capacitor 128 and resistor 129 coupled from output terminal 123 of gate 120 to input terminal 114 of gate 115.

A variable resistor 132 is coupled from gate terminal 1 14 to ground to provide a discharge path for capacitor 128. The setting of resistor 132 determines the relaxation time of the multivibrator. Diode 134 coupled across resistor 132 provides protection for the input terminal114 of gate 115. Resistor 136 couples gate terminal 113 to ground.

The output of NOR gate 115 drives a first Darlington amplifier through a resistive voltage dropping network comprising resistors 131 and 133, while the output of NOR gate 120 drives a second Darlington amplifier 139 by means of a resistive voltage dropping network comprising resistors 137 and 138 coupled as shown in FIG. 6.

Darlington amplifiers 135 and 139 are alternatively switched on by the NOR gates and, in turn, control the switching of the electronic switches 140 and shown in FIG. 5 and which comprise transistors 142 and 144 and 152 and 154 respectively. The emitters of transistors 142 and 152 are coupled to current supply node 165 while the emitters of NPN transistors 144 and 154 are coupled to ground. The collector of amplifier 135 is coupled to the base of transistor 142 while the emitter is coupled to the base of transistor 144 through resistor 143. Switch 140 provides a current path for the motor drive current indicated as I and which flows in a direction shown by the arrows in FIG. 6. When amplifier 135 is on, transistors 142 and 144 conduct current I from output terminal 165 of the variable current supply through motor 80, as shown, to rotate the head in a first direction.

Amplifier 139 has its collector terminals coupled to the base terminal of transistor 152 and the emitter terminals coupled to the base of transistor 154 through a resistor 153. When amplifier 139 is conductive, transistors 154 and 152 are turned on to conduct a drive current I which flows from node through transistors 152 and 154 in an opposite direction through motor 80 to cause the rotating head to reverse directions.

The variable D.C. supply 160 comprises an infinitely selectable constant voltage source including a series voltage dropping network comprising resistors 162 and 164 coupled from the, 12 volt supply to ground. Resistor 162 is a potentiometer having itscenter tap coupled to the base of a supply transistor 164 which has its collector coupled directly to the 12 volt supply and its emitter coupled to current output node 165 which is coupled to the emitters of transistors 142 and 152. By varying the position of the wiper arm on resistor 162, therefore, the voltage at the emitter of transistor 164 is varied to adjust the magnitude of currents I and I for varying the drive speed of motor 80 and, therefore, the scanning sector. It is noted here that since the emitter follower circuit including transistor 164 provides a selectable but constant drive voltage for motor 80, the

actual magnitude of currents l and I adjust to provide increased torque when the back EMF (electro-motiveforce) of the motor drops due to frictional loading. Thus, the scanning head speed remains nearly constant at the selected value andis self-correcting for frictional or other load variations. Having described the interconnection of the components forming the control circuit, a description of the operation thereof follows:

OPERATION For the purposes of initiating the description of the operation of the circuit, it is assumed that the motor is rotating in a direction toward the reference position (corresponding to the approximate alignment of the reed switch and magnet) after it has scanned its predetermined are measured from the reference position. The monostable multivibrator is in its stable condition. At this time, transistors 142 and 144 are conductive to supply current I, to motor 80. Also at this time, amplifier 135 is conductive since the output of NOR gate 115 is a logic I. As the magnet on the rotating head approaches the reed switch 90, the switch closes momentarily thereby impresing a positive voltage pulse on input terminal 113 of gate 115. In response to this control signal, gate 115 switches to a logic output state which output'is applied to input terminal 121 of gate 120. Gate 120 then switches to a logic 1 state at output terminal 123 which, in turn, applies a logic 1 state to input terminal 114 of gate 115 through the resistivecapacitive network 128, 129. This feedback signal maintains gate 115 in its low (i.e., 0) condition until the relaxation time has expired (i.e.-, capacitor 128 is sufficiently discharged).

When the output of gate 123 switches to the 1 state, amplifier 139 is triggered into conduction. Since gate 115 switches to a 0 state, amplifier 135 is turned off.

Thus, at the reference position, transistors 142 and 144 are switched off and transistors 152 and 154 are switched on to provide a reverse current l through motor 80 which drives the rotating head away from the referenceposition.

- 4, as is the power selector switch 112.

When the end ofthe arc is reached, gate circuit 115 switches to a logic 1 outputcondition due to the discharge of capacitor 128 and since terminal 113 is coupled to ground through resistor 136. Amplifier 135 is again turned on by the logic l at output terminal 116 and simultaneously gate circuit 120, which has the logic I applied to input terminal 121, is returned to a logic 0 output state.

Thus, amplifier 139 is turned off which shuts off transistors 152 and 154 while, at approximately the same time, amplifier 135 triggers transistors 142 and 144 into conduction to again supply current I, through motor 80 which reverses the direction of the rotating head to its first direction. The head then again rotates toward the reference position to repeat the cycle of operation. In one embodiment, the relaxation time of the multivibrator circuit was approximately 250 msec. and

the current 1 and I which are of equal magnitude, could be adjusted by resistor 162 to yield a scanning arc range from zero to in excess of 270.

It is seen that with the power switch 112 in the rotating position, input terminal 113 of NOR gate 115 is held at a 1 state, which in turn maintains output terminal 116 at a 0 state. This holds gate 120 output terminal 123 at a continuous 1 state thereby holding amplifier 139 on. Amplifier 139, in turn, actuates transistors 152 and 154 continuously to provide current I through motor for continuous rotation of the scanning head in the second direction without the reversing effect of the multivibrator. Thus, the control circuit can supply continuous rotation for general use or provide an oscillatory scan of a preselected sector which is adjustable and which has a constant scanning period.

1. A system for periodically reversing the drive direction of a reversible motor at a constant rate over a selectable range of motion by changing the drive signal applied to'the motor to vary the motor speed comprising:

a reversible drive'motor mounted to a fixed member;

actuation means positioned on a movable element driven by said motor, said element movable in op- 'posite' directions as the motor drive'signal is changed;

. detection means positioned on a fixed member to be actuated by said actuation means to develop a control signal;

monostable circuit means having a predetermined time delay and coupled to said detection means to be switched between stable and unstable states in response to said control signal therefrom;

supply means for supplying a selectable amplitude direct voltage for driving said motor at a speed selectable to determine the angle of motion through which said movable element travels during said predetermined time delay; and.

switch means coupled to said monostable circuit tobe actuated by the change of state of said monostable circuit, said switch means coupling said supply means to said drive motor in a first or-second manner to drive said motor in first or second directions inresponse to the different states of said monostable circuitf 2. The system as defined in claim 1 wherein said supply means comprises: I

a selectable constant voltage source; and means for adjusting the amplitude of voltage applied to said'drive motor by said switch means thereby varying the speed of the motor and the range of motion over which said movable element travels during a cycle of operation.

3. The system as defined in claim 1 wherein said actuation means comprises a permanent magnet and said detection means comprises a reed switch positioned such that the magnetic field from said magnet closes the contacts of said reed switch as said movable element reaches a reference position.

4. The system as defined iii claim 3 wherein said monostable circuit is a monostable multivibrator and wherein said reed switch is coupled from a voltage source to an input terminal of said multivibrator to switch said multivibrator to an unstable state as said reed switch is actuated by said magnet.

5. The system as defined in claim 4 wherein the unstable period of said multivibrator is preset to provide a relatively constant period of oscillation for said moving element.

6. .The system as defined in claim 5 wherein said switch means comprises an electronic switch including at least one input terminal coupled to an output terminal of said multivibrator and responsive to changes in state of said multivibrator to reverse the current applied to said drive motor from said voltage source.

7. A beacon sector scanning apparatus for providing an oscillatory scan of a beam of light at a substantially constant frequency and over an arc which can be adjusted, said system comprising:

stationary means including a light source for projecting a beam of light therefrom;

a rotatable head including light reflecting means, said head positioned on said stationary means to receive said beam of light from said source and project said beam through an are as said head rotates;.

reversible drive means coupled to said rotating head i to rotate said head in first and second directions;

detection means positioned on one of said head or said stationary means for generating control signals when actuated;

actuation means positioned on the other of said head or stationary means for actuating said detection means as said head reaches a predetermined position with respect to said stationary means;

a control circuit for applying a selectable constant amplitude drive signal to said'drive means for opertions, said circuit coupled to said detection means and responsive to control signals therefrom for reversing the direction of motion of said drive means for a predetermined period of time upon actuation of said detection means is actuated by said actuating means; and

means for adjusting the amplitude of said drive signal to vary the speed of rotation of said head whereby the scanning arc of said beam is varied while the scanning frequency remains substantially constant.

8. The system as defined in claim 7 wherein said stationary means comprises a base assembly and a removable support assembly rotatably supporting said rotatable head, said support assembly including a mounting means for said drive means and means for supporting said detection means thereon such that said actuation means actuates said detection means as said head approaches said detection means during the rotation thereof.

9. The system as defined in claim 8 wherein said drive means comprises a reversible electric motor and said control circuit comprises:

a monostable circuit coupled to said detection means to be switched between stable and unstable states by the actuation of said detection means to generate output signals of a predetermined duration;

an adjustable constant voltage supply; and

an electronic switch coupling said supply and said drive motor in first or second manners to actuate said motor in first or second directions, respectively, said electronic switch coupled to said monostable circuit and controlled by said output signals therefrom to apply said drive signal to said motor.

10. The system as defined in claim 9 wherein said detection means comprises a permanent magnet fixedly mounted to said head and said actuation means comprises a reed switch positioned on said stationary means such that the magnetic field from said magnet momentarily actuates said reed switch as said rotating head reaches a predetermined position relative to said stationary means. 

1. A system for periodically reversing the drive direction of a reversible motor at a constant rate over a selectable range of motion by changing the drive signal applied to the motor to vary the motor speed comprising: a reversible drive motor mounted to a fixed member; actuation means positioned on a movable element driven by said motor, said element movable in opposite directions as the motor drive signal is changed; detection means positioned on a fixed member to be actuated by said actuation means to develop a control signal; monostable circuit means having a predetermined time delay and coupled to said detection means to be switched between stable and unstable states in response to said control signal therefrom; supply means for supplying a selectable amplitude direct voltage for driving said motor at a speed selectable to determine the angle of motion through which said movable element travels during said predetermined time delay; and switch means coupled to said monostable circuit to be actuated by the change of state of said monostable circuit, said switch means coupling saId supply means to said drive motor in a first or second manner to drive said motor in first or second directions in response to the different states of said monostable circuit.
 2. The system as defined in claim 1 wherein said supply means comprises: a selectable constant voltage source; and means for adjusting the amplitude of voltage applied to said drive motor by said switch means thereby varying the speed of the motor and the range of motion over which said movable element travels during a cycle of operation.
 3. The system as defined in claim 1 wherein said actuation means comprises a permanent magnet and said detection means comprises a reed switch positioned such that the magnetic field from said magnet closes the contacts of said reed switch as said movable element reaches a reference position.
 4. The system as defined in claim 3 wherein said monostable circuit is a monostable multivibrator and wherein said reed switch is coupled from a voltage source to an input terminal of said multivibrator to switch said multivibrator to an unstable state as said reed switch is actuated by said magnet.
 5. The system as defined in claim 4 wherein the unstable period of said multivibrator is preset to provide a relatively constant period of oscillation for said moving element.
 6. The system as defined in claim 5 wherein said switch means comprises an electronic switch including at least one input terminal coupled to an output terminal of said multivibrator and responsive to changes in state of said multivibrator to reverse the current applied to said drive motor from said voltage source.
 7. A beacon sector scanning apparatus for providing an oscillatory scan of a beam of light at a substantially constant frequency and over an arc which can be adjusted, said system comprising: stationary means including a light source for projecting a beam of light therefrom; a rotatable head including light reflecting means, said head positioned on said stationary means to receive said beam of light from said source and project said beam through an arc as said head rotates; reversible drive means coupled to said rotating head to rotate said head in first and second directions; detection means positioned on one of said head or said stationary means for generating control signals when actuated; actuation means positioned on the other of said head or stationary means for actuating said detection means as said head reaches a predetermined position with respect to said stationary means; a control circuit for applying a selectable constant amplitude drive signal to said drive means for operating said drive means in first and second directions, said circuit coupled to said detection means and responsive to control signals therefrom for reversing the direction of motion of said drive means for a predetermined period of time upon actuation of said detection means is actuated by said actuating means; and means for adjusting the amplitude of said drive signal to vary the speed of rotation of said head whereby the scanning arc of said beam is varied while the scanning frequency remains substantially constant.
 8. The system as defined in claim 7 wherein said stationary means comprises a base assembly and a removable support assembly rotatably supporting said rotatable head, said support assembly including a mounting means for said drive means and means for supporting said detection means thereon such that said actuation means actuates said detection means as said head approaches said detection means during the rotation thereof.
 9. The system as defined in claim 8 wherein said drive means comprises a reversible electric motor and said control circuit comprises: a monostable circuit coupled to said detection means to be switched between stable and unstable states by the actuation of said detection means to generate output signals of a predetermined duration; an adjustable constant voltage supply; and an electronic switch couPling said supply and said drive motor in first or second manners to actuate said motor in first or second directions, respectively, said electronic switch coupled to said monostable circuit and controlled by said output signals therefrom to apply said drive signal to said motor.
 10. The system as defined in claim 9 wherein said detection means comprises a permanent magnet fixedly mounted to said head and said actuation means comprises a reed switch positioned on said stationary means such that the magnetic field from said magnet momentarily actuates said reed switch as said rotating head reaches a predetermined position relative to said stationary means. 